Evidence-appraisal glossary
Length-time bias
Length-time bias is the tendency of screening to preferentially detect slowly progressing disease, because slow cases spend longer in a detectable phase and are more likely to be caught. This makes screen-detected disease look more survivable than it is.
Also called: length bias, length-biased sampling.
Length-time bias arises because fast, aggressive disease often surfaces as symptoms between screening rounds, while indolent disease sits in the detectable window long enough to be found by a scan or test. Screen-detected cases therefore skew toward better-behaved disease, inflating apparent survival even if screening changed nothing. In its extreme form it becomes overdiagnosis, where the disease found would never have caused harm. It is distinct from lead-time bias, which is about counting survival from an earlier start point.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.